As described in HTML4 Meta data profiles and XHTML Meta Data Profile (XMDP) description:
HTML4 definition of the 'rel' attribute. Here are additional values as defined in the rel-license and rel-tag specifications.
Here are some additional values, from XFN 1.1, each of which can be used or omitted in any combination (unless otherwise noted, and except where prohibited by law) and their meanings, symmetry, transitivity and inverse if any. Please see the XFN home page for more information about XFN.
HTML4 definition of the 'rev' attribute. Here are additional values as defined in the vote-links specification .
HTML4 definition of the 'class' attribute. This meta data profile defines some 'class' attribute values (class names) and their meanings as suggested by a draft of "Hypertext Links in HTML". All values are defined according to the semantics defined in the hCard (based on RFC 2426), hCalendar (based on RFC 2445), and XOXO specifications.
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